Lose heat when idling

[Tequila Man]

I have a 91 Blazer with the 6.2l NA. I am curious about my heating system. I get absolutely no heat until the thermostat seems to open. When it opens the temp gauge shoots up to almost red line then drops super fast. That is when I get heat. It comes in all of a sudden. Then it is fine till I stop. If I idle for more than a minute or so, I lose my heat. The temp guage seems normal. I put in a new thermostat, flushed the heater core and, even a new water pump.
Can anyone help me with this?

[simon]

With heat you mean the cab heater right?
If your temp gauge shoots up like you say , i would check that new thermostat, mayby you put it in upside down? check it in a pan of boiling water if it works ok. check your temp sensor, and the heater valve could be stuck too
it is probably located in the heater supply hose.

good luck

[darkroad]

A couple of things come to mind. If you have drained or changed antifreeze lately some engines get an air lock in them and are hard to get it burped out. This could create a hot area in the heads so that when your thermostat does open it get a big shot of almost steamy fluid pushed out of the heads and thru the system. I had this happen once to my 1990 Suburban 6.2 NA, it took a while but all at once a couple of weeks later it cleared itself out and I added more to the radiator.

Another thing you need to do is change the return hose configuration of the heater hose. Mine hardly would get any cab heat until the thermostat heated up, about 6 miles down the road. Take the line that returns out of the heater core and reroute it back into the water pump and block off the
inlet in the radiator with a short piece of hose with a small piece of black iron pipe with a cap on it. Use hose clamps on both ends. You will be amazed that after only about 1/4 to 1/2 half mile down the road you now have heat starting to come out of the ducts. If you do a search of the forums I think there is even a couple of photos showing it. I did have to use a 45 degree
pipe fitting at the pump to get a better angle as the hose now comes out between the pump and the bottom of the alternator.



Darkroad